SEXUAL ACTIVITY OF MALE RABBITS Sie 
services daily. He also states that bulls have been used on 400 
cows in a year, and that even poorly fed bulls will make from 
four to eight copulations daily, and this without bad effect. 
Strictly speaking, ‘vitality’ of individuals cannot be measured, 
for the vitality of any individual really means the sum total of 
life force within every living cell of the organism; vitality, as 
used in speaking of animals, may, however, in part be measured 
by the rate of growth in weight, the skeletal development, and 
the ability of the individual to live to a good age. Such factors 
as body weight and the others mentioned above are measurable. 
The purpose of this investigation has been to study the effects of 
heavy service of males on the nature of their offspring, as far as 
we could measure the effect. 
MATERIAL AND METHODS 
1. Animals used 
The character of the animals used in this investigation has 
been discussed to some extent in the first paper of this series. 
Stocks of the European domestic rabbit, Lepus cuniculus, secured 
from six different breeders, were used and no inbreeding was 
practiced at any time. The weight! and age of the females is an 
important factor in that it affects both the number in a litter and 
the individual weights of the offspring. Likewise the weight of 
the male probably is of much importance in affecting the weight 
of the young. The maturity of the male is a factor that should 
not be lost sight of, because all three of the males used were fully 
mature and were in their prime of life—about two years old. 
The average weights of the males are as follows: No. 1, 2850 
grams; No. 3, 2575 grams; and No. 4, 2200 grams. 
Shy breeders sometimes occur in rabbits, but most of these 
females proved to be regular breeders. No. 25, however, was 
barren and was discarded; No. 12 produced young three times, 
the last time August 5, 1916, after which time she appeared 
never to come in heat again and continually refused to copulate 
1 Prof. H. W. Vaughan has found that Large Type Poland-China Swine pro- 
duce larger litters than the Small Type. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 25, NO. 2 
